In the field of engines used in vehicles, for instance, a widely-known exhaust turbocharger rotates a turbine by energy of exhaust gas of an engine, then compresses intake air by a centrifugal compressor directly connected to the turbine via a rotational shaft, and supplies the compressed air to the engine in order to improve the output of the engine.
As represented by the normal compressor of the performance-characteristic comparison chart in FIG. 10 where y-axis is the pressure ratio and x-axis is the flow rate, a compressor of such an exhaust turbocharger is stably operated in the flow-rate range from a surge flow rate (left-hand line in the drawing) at which surging, or pulsation of the entire system, occurs, to a choke flow rate (right-hand line in the drawing) at which choking occurs and the flow rate stops increasing.
However, in a centrifugal compressor of the normal compressor type in which intake air is directly introduced into an impeller wheel, the flow-rate range between the choke flow rate and the surge flow rate where stable operation is possible is narrow. Thus, there is a problem in that it is necessary to operate the compressor at an inefficient operation point which is differed from the surge flow rate, in order to avoid surging.
In order to solve the above problem, Patent Document 1 discloses a technique of increasing the operation range of an exhaust turbocharger by providing guide vanes at the upstream side of an impeller wheel of the centrifugal compressor to swirl intake air at the upstream side of the impeller wheel, and a technique of providing a recirculation flow path for a housing of a supercharger to recirculate a part of intake gas introduced into the impeller wheel.
Such techniques will be described briefly in reference to FIG. 9.
An impeller wheel 201 of a centrifugal compressor 200 includes a plurality of vanes 204 which are rotatable in a housing 202, and the housing 202 includes an inner wall disposed in the vicinity of radially outer edges 204a of the vanes 204.
An intake-gas inlet of the centrifugal compressor 200 includes an outer annular wall 207 forming a gas inlet 208, and an inner annular wall 209 extending inside the outer annular wall 207 to form an inducer part 210. An annular gas flow channel 211 is formed between the annular walls 209, 207.
A housing surface 205 by which the vanes 204 pass through is in communication with the annular flow channel 211 via a downstream opening part 213.
An upstream opening part brings the annular flow channel 211 into communication with the inducer part 210 being the inlet intake part. Inlet guide vanes 214 are provided inside the inducer part 210 downstream with respect to the upstream opening part to induce precedent swirls in the gas flow passing through the inducer part 210. When the flow rate of the air passing through the compressor is small due to the above configuration, the direction of the air flow passing through the annular flow channel 211 is reversed, and the air flows from the impeller wheel through an opening 213 and the annular flow channel 211 extending in the upstream direction to be introduced again into the gas inlet 208, so as to recirculate in the compressor.
As a result, performance of the compressor is stabled, and the compressor surge margin and the choke flow are both improved (see “RCC (recirculation compressor)” of FIG. 10).
Further, Patent Document 1 discloses that the inner annular wall 209 and the outer annular wall 207 extend in the upstream direction and house an inlet guide-vane apparatus. The inlet guide-vane apparatus includes a plurality of the inlet guide vanes 214 extending between a center nose cone 215 and the inner annular wall 209.
The inlet guide vanes 214 sweep forward in the rotational direction of the impeller wheel 201 to induce precedent swirls in the air flow which reaches the impeller wheel 201. The precedent swirls improve the surge margin (surge limit) of the compressor. In other words, the precedent swirl flow reduces the flow which causes surging in the compressor. (see the “RCC with guide vanes” of FIG. 10).